Graft polymerization on polymeric substrates



Nov. 21, 1967 E. WOLINSKI 3,353,933

GRAFT POLYMERIZATION ON POLYMERIC SUBSTRATES Filed May 20, 1964 ORGANIC POLMERIC FILM WATER ETHYLENICALLY WASH UNSATURATED AQUEOUS HYDROGEN ELECTRICAL DISCHARGE TREATER HONOHER PEROXIDE INVENTOR LEON EDWARD WOLINSKI ATTORNEY Patented Nov. 21, 1967 3,353,988 GRAFT POLYMERIZATION ON POLYMERIC SUBSTRATES Leon Edward Wolinski, Buffalo, N.Y., assignor to E. I. du

Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed May,20, 1964, Ser. No. 368,892

. 4 Claims. (Cl. 11747) ABSTRACT OF THE DISCLOSURE It is known to produce graft polymers on organic polymer substrates which have been subjected to the action of high energy ionizing radiation. The exposure of organic polymers to high energy radiation, however, frequently involves .many side effects including cross linking and degradation which, in turn, alter the physical properties of-the irradiated polymer.

It is also known to effect graft polymerization on organic polymer substrates by subjecting such a substrate in the substantial absence of oxygen to relatively low .energy ionizing radiation. While such treatments avoid the adverseactions of the high energy irradiation on such substrates, the process tends to be slow by present day commercial. standards, and provisions have to be made for the elimination or avoidance of an oxygen.

It is therefore an object of this invention to provide an economically feasible'process, free of degradative side atmosphere of elTects",for-graft polymerization of polymerizable monomers onto polymeric films. The foregoing and related ends will more clearly appear from the detailed description .which follows. i The objects are realized by the process of the present invention which, briefly stated, comprises in combination the sequentialsteps of (1) subjecting an organic poly- -mer ic shaped structure, eg, organic polymeric film, to -the action ofan electrical discharge at substantially atmospheric pressure between spaced electrodes maintained at a voltage-differential in excess of about 1000 volts said electrical discharge having an average energy level below 15 electron volts; (2) contacting the surface of the resulting polymericshaped structure with an agent capable of providing asource of free radicals; and (3) substantially immediately thereafter contacting the surface of the polymeric'shapedstructure with an ethylenically unsaturated monomer whereby to form on said surface a polymeric coating.

Because of the commercial importance of organic polymeric films this invention will be hereinafter ,described with specific reference to such shaped structures as the substrate material. It will be apparent, however, that the principles of this invention are applicable to all organic polymeric shaped structures including in addition to film, filaments, woven and non-woven fabrics, ribbons, rods, tubes, etc.

In a preferred embodiment of this invention, illustrated diagrammatically in the accompanying drawing, a thermoplastic organic polymeric film is continuously passed between a set of spaced electrodes consisting of a rotating metal roll which is connected electrically to ground and one or more stationary electrodes disposed parallel to the longitudinal axis of the roll and spaced a distance of from 0.03 to 0.125 of an inch from the surface thereof. The electrodes are each connected to a suitable power source which supplies an alternating current of from 0.3 to 5.5 RMS (root mean square) amperes at a voltage in the range of 10,000 to 30,000 volts with pulsating peak voltages up to 100,000 volts and at a frequency of at least 350 cycles per second and preferably in the range of 300,000 to 500,000 cycles per second. The film is then continuously passed thru a bath consisting of an aqueous solution of 30% hydrogen peroxide, which provides a source of free radicals thence through a water wash bath to remove excess hydrogen peroxide from the surface of the film, and finally is drawn thru a bath of ethylenically unsaturated polymerizable monomer which polymerizes in situ to form on the surface of the film exposed to the electrical discharge a polymeric coating which is chemically bonded to the substrate.

As indicated above the preferred agent capable of providing free radicals is hydrogen peroxide. In addition to hydrogen peroxide agents such as benzoyl peroxide, N- nitrosoacylanilides, para-bromobenzenediazo hydroxide, triphenylmethylazo benzene, azobisisobutyronitrile and tetraphenylsuccinonitrile are typical of other agents which may be employed with like effect. It is generally preferred to wash excess agent from the surface of the film after treatment. However, grafting, i.e. chemical bonding, of the polymer coating to the substrate is afiected also when the washing step is omitted but the yield of grafted polymer may be lower.

The process of this invention is effective to provide an adherent coating of polymeric material on any substrate of organic polymer of the thermoplastic or thermosetting type. Among the polymeric substrates, e.g., films, which may be submitted to the grafting techniques described in this invention are the hydrocarbon polymers such aspolyethylene, polystyrene, polybutadiene, rubber, polyisobutylene, butadiene/styrene copolymers and the like; halogenated hydrocarbon polymers such as polyvinyl chloride, polyvinylidene chloride, polychlorene, polytetrafiuoroethylene, polyvinyl fluoride, chlorinated and chlorosulfonated polyethylene and the like; ester-containing polymers such as polyvinyl acetate, polymethyl methacrylate, polyethylene terephthalate and the like; hydroxyl containing polymers such as polyvinyl alcohol, cellulose, regenerated cellulose and the like; ether containing polymers such as polyethylene oxide, polymeric formaldehyde, solid polyethylene tetra'hydrofurane, dioxalene polymers and the like, condensation polymers such as the polyamides, polyimides, phenyl/formaldehyde polymers, urea/formaldehyde polymers, triazine/formaldehyde pol- .ymers 'and the like, polypeptides, silicones, and olefin polysulfones.

As examples of ethylenically unsaturated monomers which can be grafted onto the various polymeric substrates, may be mentioned alkyl acrylates and methacrylates such as methyl methacrylate, ethyl acrylate, etc.,

inch to as much as 0.25 inch provided suitable adjustments in such features as amount of current, electrode dimension and exposure time are made.

The following specific examples will serve to further possible, consistent with effective treatment would normally be employed.

Preferably, the electrodes are spaced from about .03 inch to about 0.125 inch apart. However, useful results I can be obtained when the electrodes gap is as low as .015

acrylic and methacrylic acid, ethylene and its derivatives 5 illustrate the principles and practice of this invention. such as the halosulfonated ethylenes and tetrafiuoroethyl- In each of the examples tabulated below a film. of the ene, vinyl chloride, vinyl acetate, vinylidene chloride, stycharacter specified was drawn through the electrical disrene, acrylonitrile, methacrylonitrile, butadiene and isocharge apparatus of the type illustrated in the accompany! prene. ing drawing at a rate of feet per minute. The electrodes In some cases, in order to realize more efficient graft- 10 of the apparatus were connected to a Lepel High Frequening, it may be desirable to protect the film after the steps cy Spark Generator Model HFSG (Lepel High Frequency of activation in the electrical discharge and exposure to Laboratory). The setting on the unit was 55 for each electhe free radical source with an inert blanketing atmostrode. The film after treating in the electrical discharge phere such as steam, carbon dioxide or nitrogen. was then drawn through an aqueous solution of 30% hy- Any suitable means known to the art for providing an drogen peroxide containing a small amount of sodium electrical stress field (i.e., an electrostatic field) of alteroleate as a wetting agent. The film was thereafter drawn nating or pulsating character between spaced electrodes through a wash tank to remove excess hydrogen peroxide (e.g. a high frequency spark generator of the type hereinfrom the film surface and the resulting Washed film was after identified or a motor generator setup) may be emthen drawn through a solution of the specified ethylenicalployed as the power source in the electrical discharge ly unsaturated monomer. The resulting coated film was treatment of this invention. Such source should provide then subjected to a number of tests designed to dete t difa potential at the electrode which is within the range varyferences in the characteristics of the film over that of the ing from very low voltages in the order of about 1000 untreated film. Such differences or improvements are revolts up to pulsating peak of 100,000 volts and above. corded under the heading Remarks in Table I below.

TABLE I Example Polymeric Film Monomer Remarks 1 Branched polyethylene Vinylidene chloride Layer of coating formed on surface. Coating layer essentially unaffected by extraction with methyl ethyl ketone. Coated film had lower moisture permeability than nontreated filrn. 2 Linear polyethylene Styrene Glossy layer formed on surface. Treated film had increased electrical surface resistance. Extraction with chloroform 'did not remove 3 Biaxially oriented polypropy- Vinylidene chloride 'IESSt ek l fiIm had layer of coating and showed decreased oxygen lene. permeability. Coating not removed on extraction of the film with methyl ethyl ketonc. 4 Biaxially oriented polyethylene Methyl rnethacrylate Coating formed on surface. Coated filrn showed better surf ce slip terephthalate. than nontreatcd film. Coating was not removed by extraction 5 Biaxially oriented linear Vinyl chloride/vinylidene Tr e a t ecl iil fh gl iowed strong adhesion to poly-amide-topcoating. Toppolyethylene. chloride mixture. coating could not be peeled-from base layer without tearing of base.

' Topcoat stripped easily from nontreated control film. 6 Polyhexarnethylene adipamide- Vinyl acetate Treated product could be readily heat-sealed, nontreated control could not be heat sealed at normal sealing temperatures.

Frequencies from 350 cycles per second up to 500,000 It will be evident from the foregoing description and cycles per second or higher can be used and frequencies examples that the process of this invention provides a in the range of 300,000 to 500,000 cycles are preferred for simple and economic expedient for providing an organic rapid and effective treatment. polymeric substrate, and particularly organic polymeric, In general, the effectiveness of the treatment increases self-supporting film structures with a polymeric continu- With amount of current to the electrodes for a given area ous coating selected to enhance the properties and utility of electrode and time of exposure. Current to the elecof the substrate. trodes may range up to 5.5 RMS (root mean square) am- I claim: peres or higher. However, it is preferred to operate in the 1. A process which comprises, in combination, the serange of 0.3 RMS amperes to 3.5 RMS amperes to give quential steps of (1) subjecting an organic polymeric reasonable treating times on the one hand, and to avoid shaped structure to the action of an electrical discharge in rapid degradation of the electrodes at too high currents on air at substantially atmospheric pressure between spaced .the other-hand. Power to the high frequency generator may electrodes maintained at a voltage differential in-excess of range from 10 watts per lineal inch of the electrode length about 1000 volts, said electrical discharge having an avto 1800 watts per lineal-inch of the electrode length. The erage energy level below 15 electron volts; (2-) contacting electrical discharge employed herein, operating Within the the surface of the resulting polymeric shaped structure parameters above specified, has an average energy level with an agent capable of providing a source of free radibelow 15 electron volts, and is not to be confused with cals;'and (3) substantially immediately thereafter contactthe high or intermediate energy irradiations heretofore ing the surface of the polymeric shaped structure with an used to treat polymeric surfaces. ethylenically unsaturated monomer whereby to form on Time of exposure to the electric discharge treatment is said surface an adherent polymeric coating. not especially critical and effective treatments are realized 2. The process of claim 1 wherein said agent is an at exposure times as low as one second or less and no adaqueous solution of hydrogen peroxide. verse effects are noted at times as long as 60 seconds. 3. The process of claim 1 wherein said shaped struc- Longer exposure times can probably also be employed alture is a self-supporting film. though for economic reasons exposure times as short as 4. A process which compr-ises, in combination, the sequential steps of (1) passing, at substantially atmospheric pressure, a continuous thermoplastic organic polymeric film continuously betweenaset of spaced electrodes consisting of a rotating metal roll connected electrically to ground over whiQh the film passes, and at least one elon- 5 gated stationary electrode aligned with its longitudinal axis parallel to the longitudinal axis of the roll and spaced a distance of from 0.015 to 0.25 of an inch from the surface thereof, while continuously applying to said stationary electrode an alternating current at a voltage within the range of from 1000 to 100,000 volts, and at a frequency in the range of 350 to 500,000 cycles per second, to create an electrical discharge in air between said electrodes having an average energy level below 15 electron volts; (2) then continuously passing said film thru a bath consisting essentially of an aqueous solution of hydrogen peroxide; and (3) continuously passing said film thru a bath of ethylenically unsaturated monomer whereby t0 form on the surface of the film subjected to the action of said electrical discharge an adherent polymeric coating.

References Cited UNITED STATES PATENTS 2,802,085 8/1957 Rothacker 117-47 2,907,675 10/ 1959 Gaylord 11747 2,935,418 5/1960 Berthold et a1 l1747 2,955,953 10/1960 Graham 117-47 10 3,111,424 11/1963 Le Clair 1l747 MURRAY KATZ, Primary Examiner. 

1. A PROGRESS WHICH COMPRISES, IN COMBINATION, THE SEQUENTIAL STEPS OF (1) SUBJECTING AN ORGANIC POLYMERIC SHAPED STUCTURE TO THE ACTION OF AN ELECTRICAL DISCHARGE IN AIR AT SUBSTANTIALLY ATMOSPHERIC PRESSURE BETWEEN SPACED ELECTRODES MAINTAINED AT A VOLTAGE DIFFERENTIAL IN EXCESS OF ABOUT 1000 VOLTS, SAID ELECTRICAL DISCHARGE HAVING AN AVERAGE ENERGY LEVEL BELOW 15 ELECTRON VOLTS; (2) CONTACTING THE SURFACE OF THE RESULTING POLYMERIC SHAPED STRUCTURE WITH AN AGENT CAPABLE OF PROVIDING A SOURCE OF FREE RADICALS; AND (3) SUBSTANTIALLY IMMEDIATELY THEREAFTER CONTACTING THE SURFACE OF THE POLYMERIC SHAPED STRUCTURE WITH AN ETHYLENICALLY UNSATURATED MONOMER WHEREBY TO FORM ON SAID SURFACE AN ADHERENT POLYMERIC COATING. 